The present invention relates to a method for filling small via holes provided to insulating film to expose parts of the underlayer on a wafer by metal by means of selective CVD (chemical vapor deposition) and an apparatus therefor. In particular, it relates to a method for via filling by metal suitable for filling small via holes by metal which is capable of both securing a satisfactory selectivity and providing a good low contact resistance, and to an apparatus therefor.
With the recent trend toward more highly integrated LSIs, difficulties in wiring design for connection between elements and wiring or between respective wirings are becoming more serious, and multilayer wiring, so called multilevel metallization, has become an indispensable technique for overcoming the difficulties. In order to connect a lower layer wiring and an upper layer wiring provided thereon with an insulating film provided therebetween, there is used a method which comprises providing minute via holes to the insulating film and filling the via holes by a metal. Several methods are known for filling via holes. Among them, one of the most promising methods for use in practice is selective CVD of a metal (particularly tungsten) because it shows a good via filling ability even when the diameter of via hole is very small.
The method of selective CVD of tungsten comprises introducing a mixture of tungsten fluoride (WF.sub.6) gas and hydrogen (H.sub.2) gas over a sample heated to above 250.degree. C. and making them contact with each other, and thereby making a tungsten (W) film grow on the underlayer metal (exemplified herein by aluminum) based on either of the following reactions. EQU WF.sub.6 +2Al.fwdarw.W+2AlF.sub.3 ( 1) EQU WF.sub.6 +3H.sub.2 .fwdarw.W+6HF (2)
On an insulating film such as SiO.sub.2 film, the reaction (1) does not take place and also the reaction (2) does not proceed at a temperature not higher than 700.degree. C., so that tungsten grows selectively on aluminum, whereby via filling can be achieved.
References which give description of selective CVD of tungsten include J. Electrochem. Soc. 131, 1427-1433 (1984) and Proc. of VLSI Multilevel Interconnection Conference (Jun. 15-16, 1987) p 132-137. However, even when the methods described in above references are used, such insulating substances as oxide film present on Al underlayer and AlF.sub.3 formed by the reaction (1) remain at the interface between tungsten and aluminum in the via hole, making it difficult to obtain a sufficiently low contact resistance at the via hole part. Methods which have been proposed to solve the above problem include one comprising forming the tungsten film while heating the wafer above 380.degree. C. as disclosed in Proc. of VLSI Multilevel Interconnection Conference (Jun. 15-16, 1987) p. 208-215, and a method comprising attaching a thin (about 500 .ANG. thick) MoSi.sub.2 film onto aluminum and thereby inserting MoSi.sub.2 between aluminum and tungsten, whereby the tungsten film can be formed without leaving aluminum surface oxide film and AlF.sub.3 behind at the interface, as disclosed in Toshiba Review, Vol. 41, No. 12, p. 988-991.
Recently, a method has been reported in which SiH.sub.4 or a similar gas is used in place of H.sub.2 as a reducing gas, as described, for example, in Technical Digest IEDM (1987), P213-P219. The use of this method enables a high rate film growth at a wafer heating temperature as low as 250.degree.-320.degree. C. In this method, at a temperature not below 340.degree. C., the selectivity is lost and selective via filling cannot be achieved.
In the prior techniques mentioned above, however, no sufficient consideration is given regarding the treatment of underlayer metal surface on which tungsten is to be grown by selective CVD, resulting in the following problems. That is, the contact resistance at the via hole is not-sufficiently low or, even when the contact resistance at the via hole is low, the resistance of wiring itself increases; or tungsten grows also on the insulating film as the result of surface cleaning treatment of via hole underlayers, leading to occurrence of short circuit between adjacent through holes.
The surface of underlayer metal after formation of small via holes is contaminated by fouling originated from a photoetching process applied for providing the small via holes, or the metal surface is formed of a oxide film (Al.sub.2 O.sub.3 etc. when the underlayer metal is aluminum, for example) purposely provided as an anti-corrosive treatment against a halogen-containing gas used as an etching gas. Therefore, the underlayer has no clean metal surface exposed, and impurities which increase contact resistance remain at the interface between underlayer metal and tungsten even after tungsten film has been grown. The oxides etc. which remain at the interface are, when a wafer temperature of 380.degree. C. or more is used in growth of tungsten, sometimes decreased in amount through the etching reaction of WF.sub.6 and diffusion within film during heating, resulting in a sufficiently low contact resistance. However, when the underlayer surface conditions of small via holes differ from wafer to wafer, a sufficiently low contact resistance cannot always be obtained with good reproducibility. To solve the above problems, namely poor reproducibility of low contact resistance and increased contact resistance of the tungsten/aluminum interface formed by the selective CVD of tungsten mentioned above, there has been proposed a method which uses as an underlayer an aluminum of a laminate film formed by attaching a MoSi.sub.2 film (about 500 .ANG. thick) onto aluminum. In this method, since the remaining oxygen at the interface is decreased by making the exposed part of the small via hole constituted of MoSi.sub.2 more difficultly oxidizable than aluminum and further since the formation of insulative AlF.sub.3 with a low vapor pressure according to the above-mentioned equation (1) does not take place at the interface, the contact resistance of the W/MoSi.sub.2 /Al is lower than that of the W/Al interface. However, this method is accompanied by a problem of requiring etching for forming a wiring of laminated film of MoSi.sub.2 and Al and of increasing wiring resistance due to the use of MoSi.sub.2 having a high resistivity. The above-mentioned increase in resistance poses no problem in the case of MOSLSI such as DRAM and SRAM because the film thickness of MoSi.sub.2 is very thin as compared with that of aluminum. In the case of such LSIs as bipolar and biCMOS which capitalize on their high speed, however, even a slight increase in resistance raises a serious problem.
On the other hand, several methods have been tried in which aluminum is used as the underlayer and the via hole is filled by tungsten after cleaning the surface of the underlayer (including removal of Al.sub.2 O.sub.3 on the aluminum surface). Methods used for cleaning the underlayer surface include a wet etching which uses a solution containing hydrogen fluoride (HF) or a compound thereof such as ammonium fluoride (NH.sub.4 F) and a sputter-etching which uses Ar ions. In the former treatment, however, a slight amount of fluorine remains even after washing and drying steps and causes the corrosion of aluminum underlayer. The latter sputter-etching treatment enables exposure of a clean underlayer surface since it physically removes the outer surface of the underlayer, and is hence in use as the method of pretreatment of underlayers in multilevel interconnection of sputtered aluminum. In this method, however, it has been revealed as described below that selectivity is lowered in the selective CVD, caused by the fact that the insulating film is sputter-etched simultaneously. When an insulating film is sputter-etched, the composition of the surface layer of insulator changes owing to the difference in sputtering yield between elements. In a SiO.sub.2 film, for example, since O atom is more susceptible to sputtering than Si atom, the surface layer comes to have a composition rich in Si. In other words, active si atoms come to exist at the insulating film surface. This phenomenon has been studied by means of X-ray photoelectron spectroscopy (XPS or ESCA) and discussed, for example, in J. Vac. Sci. Technol., A 3(5) (1985), pp 1921-1928 and J. Phys. D: Applied Phys., 20 (1987), pp 1091-1094.
When the selective CVD of tungsten is carried out under such conditions, the growth of tungsten proceeds presumably as the result of the following reaction: EQU WF.sub.6 3/2Si.fwdarw.W+3/2SiF.sub.4 ( 3)
Accordingly, tungsten grows also on SiO.sub.2, resulting in lowering of selectivity. This applies also to the selective CVD of other metals than tungsten. Thus, since selective CVD is based on the difference in chemical activity between respective surface parts, when a part at which no growth is desired, namely the surface of insulating film, is sputtered and activated, selectively is lowered resultantly. When a metal grows on insulating film, it gives rise to a possibility of short circuit with adjacent through holes and, further, since the metal film formed on the insulating film is apt to peel off, it remains as dirt on the wafer and causes the lowering of yield.
In the foregoing, description was made with reference to a via hole using an aluminum wiring as an underlayer because the surface oxide film on aluminum are generally known as the representative of the most difficultly removable materials. However, in case of via filling of a via hole with Si contact, which is called a contact hole occasionally, where doped Si, various barrier metals (e.g., WSi.sub.2, MoSi.sub.2, TiSi.sub.2, PtSi, and TiW) are used as underlayer, there arise problems different from those in via holes using an aluminum wiring as an underlayer mentioned above. Tungsten film is formed relatively easily on the surface of the above-mentioned materials which may possibly constitute the underlayer of contact holes, because they do not form an oxide film so strongly as the aluminum does. However, in case that a number of different materials come to exist as the underlayer of the bottom part of holes, the growth rate of tungsten varies depending on the difference of materials of underlayer, leading to a situation wherein while via filling by tungsten has been completed in holes of a certain underlayer, the growth of tungsten has just begun at other holes. This is attributable to the differences in thickness, and/or quality of the oxide film present on the surface of underlayers caused by the difference in material of underlayers. It is generally considered that in W-CVD (tungsten chemical vapor deposition) the growth of tungsten does not begin simultaneously with the introduction of raw material gas, but an induction time is present after the introduction of gas till the substantial initiation of growth of tungsten, because the surface oxide film of the underlayer delays the initiation of tungsten film growth. Therefore, the presence of contact holes different in thickness and quality of the surface oxide film of underlayers on the same wafer leads to difficulty in obtaining uniform film thickness in via filling by tungsten. Thus, it is necessary to remove these surface oxide films of underlayer in order to obtain a uniform film thickness of via filling by tungsten in contact holes. In doped Si and various silicides used as the underlayer material of contact holes, wet etching treatment with a solution containing hydrogen fluoride or a compound thereof such as ammonium fluoride is carried out as a means for cleaning the underlayer surface and the surface oxide film is removed thereby because no problem of corrosion arises unlike in aluminum. However, even in a wafer subjected to such wet etching treatment, there remains a problem unsolved in that a surface oxide film is formed during drying wafers or during the time preceding the transport to a CVD apparatus, resulting in not uniform film thickness in via filling by tungsten. On the other hand, when the sputter-etching of a surface oxide film of the underlayers and the growth of tungsten film are carried out continuously, no oxide film is formed on the underlayer surface of the contact hole bottom before tungsten film growth but, as described above with reference to via holes on aluminum wiring, there arises the problem of decreased selectivity.